Weathering performance of rigid PVC pigmented with iron‐containing pigments: The relationship between weathering and the chemical nature of the iron contained

Weathering performance of rigid PVC pigmented with iron oxide or some complex inorganic pigments (CIP) containing iron can be poor. Consideration of iron‐containing CIP's for use in exterior PVC leads to the common question: “How much ‘free iron’ is present in the pigment?” It has been unclear what “free iron” means and therefore difficult to know which iron‐containing pigments will weather well and which will not. Some formulators may avoid iron‐containing pigments altogether by electing not to formulate in a color space only achievable with those pigments. We compare the weathering performance of a series of iron‐containing CIP's to the amount of iron contained, the crystal phases, and the amount of acid‐extractable iron. We see a strong correlation between the weathering performance and the extractable iron content. Many iron‐containing pigments have good weathering performance in PVC, so one shouldn't exclude iron‐containing pigments from consideration based solely on the iron content. The acid‐extractable iron is an indicator of the expected weathering performance. Careful evaluation and selection of pigments may allow consideration of previously avoided pigment combinations.